This invention relates to a method of controlling the quantity of fuel being supplied to an internal combustion engine when the engine is in a cold state.
A fuel supply control method for internal combustion engines has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 57-137633, which is adapted to control the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine by electrically controlling the valve opening period of a fuel injection valve through which fuel is supplied to the engine, that is, by controlling the fuel injection quantity.
According to this proposed fuel supply control method, the valve opening period of the fuel injection valve is determined by adding values of various correction variables such as an intake air temperature-dependent correction variable and a warming-up fuel increasing correction variable to and/or multiplying thereby a basic value of valve opening period corresponding to the enging rotational speed and a parameter representing the engine load, e.g. intake pipe absolute pressure.
Since the above basic value is set based on air density at a predetermined reference value of intake air temperature (e.g. 30.degree. C.), the intake air temperature-dependent correction variable is used to correct the basic value in order to compensate for a change in the air density caused by deviation of the intake air temperature from the predetermined reference value. On the other hand, since there can be a difference between the quantity of fuel injected and that actually drawn and burnt in the cylinder, depending upon the atomization degree of injected fuel and the quantity of the injected fuel adhering to the wall of the intake pipe, the warming-up fuel increasing correction variable is used to correct the basic value to compensate for the difference.
The warming-up fuel increasing correction variable is determined based not only on engine temperature, e.g. engine cooling water (coolant) temperature, but also on the intake pipe absolute pressure, because, even if the engine temperature remains unchanged, a change in the intake pipe absolute pressure, i.e., a change in the flow rate of air in the intake pipe can result in a corresponding change in the quantity of fuel adhering to the intake pipe wall as well as a change in the fuel atomization degree.
However, the atomization degree of injected fuel also varies as a function of the intake air temperature, too, and hence a further correction with regard thereto is required. In particular, when the intake air temperature is low, it is difficult for the conventional fuel supply control method to secure the supply of such a proper quantity of an air-fuel mixture to the engine as to obtain stable combustion and stable engine rotation, thus suffering from degradation in the driveability of the engine, etc.